lung (n.) Look up lung at Dictionary.com
"human respiratory organ," c.1300, from Old English lungen (plural), from Proto-Germanic *lungw- (cognates: Old Norse lunge, Old Frisian lungen, Middle Dutch longhe, Dutch long, Old High German lungun, German lunge "lung"), literally "the light organ," from PIE *legwh- "not heavy, having little weight; easy, agile, nimble" (cognates: Russian lëgkij, Polish lekki "light;" Russian lëgkoje "lung," Greek elaphros "light" in weight; see also lever).

The notion probably is from the fact that, when thrown into a pot of water, lungs of a slaughtered animal float, while the heart, liver, etc., do not. Compare also Portuguese leve "lung," from Latin levis "light;" Irish scaman "lungs," from scaman "light;" Welsh ysgyfaint "lungs," from ysgafn "light." See also lights, pulmonary. Lung cancer attested from 1882.